Semiconductor device or chip bonding involves attaching the chip to a substrate with a layer of adhesive. The substrates may be as diverse as ceramic chip carders, organic printed wiring boards, copper heat sinks or Kovar lead frames. Typically, the adhesive does not serve an electrical function, or is at most a grounding contact. The functionality of the adhesive is therefore primarily defined by thermal and mechanical rather than electrical requirements. In order to provide for a highly reliable attachment of the semiconductor device to the substrate, the adhesive must be an excellent thermal conductor and also must approximate the thermal expansion coefficient of the chip. The thermal expansion mismatch between the chip and the substrate is absorbed primarily by the adhesive, making it susceptible to fatigue fracture or disbonding, sometimes transmitting stresses to the die. It would be highly desirable to formulate a die attach adhesive that has excellent thermal conductivity and a low thermal expansion coefficient.
In order to improve thermal conductivity and reduce the expansion coefficient, particulate fillers are added to the adhesives. Greater increases may be obtained by the incorporation of metallic fibers or other materials with improved thermal conductivity. Increasing the volume loading of fillers also results in a proportional increase in the thermal conductivity. If metallic powders or fibers are undesirable from an electrical standpoint, it has been shown that significant increases in thermal conductivity may be obtained using non-conductive materials such as silicon carbide or beryllium oxide. Because of their cost and toxicity considerations, these materials are seldom used. Inert silica materials are commonly used in electronic packaging adhesives. For other applications, you find calcium carbonates, calcium silicates, talcs, micas and even clay incorporated into the resin. Silica or silver particles are currently the primary fillers used in die attach adhesives. Although it provides excellent thermal conductivity, silver does not achieve the desired low thermal expansion coefficient. Silica, on the other hand, is used in adhesives in order to create a low thermal expansion coefficient material, but does not achieve the thermal conductivity that may be obtained through the use of silver.
As a result, a need exists for a lower cost filler material that can be used to formulate adhesives with improved thermal conductivity and lower thermal expansion coefficients.